1. Technical Field
The present invention relates to a shift control device for a continuously variable transmission installed in a vehicle.
2. Related Art
Continuously variable transmissions (CVTs), such as chain (or belt) type CVTs and toroidal type CVTs, which can steplessly change the gear ratio, have come into widespread use in recent years as automotive automatic transmissions. A chain type continuously variable transmission includes a primary pulley provided to an input shaft, a secondary pulley provided to an output shaft, and a chain serving as a drive transmission component, wound over the pulleys. The gear ratio is steplessly changed by changing the groove width of the pulleys, thereby continuously changing the diameter of the position where the drive transmission component rides.
The gear ratio of such continuously variable transmissions is controlled in accordance with parameters indicating the running state of the vehicle, such as accelerator position and vehicle speed, for example. This means that target turbine speed (or target engine speed, target primary pulley speed) are set based on these parameters, and the gear ratio is controlled such that the actual turbine speed converges on the target turbine engine speed.
Such continuously variable transmission sometimes has a downhill control function to correct the gear ratio of the continuously variable transmission to a low side (downshift direction) when driving downhill (see Japanese Unexamined Patent Application Publication (JP-A) No. 2003-83434, for example).
A control device of the continuously variable transmission described in JP-A No. 2003-83434 obtains a running resistance increase amount ΔR, with running resistance for the vehicle when running on a flat road as a reference. When the running resistance increase amount ΔR is a negative value, the controller determines that the vehicle is traveling downhill, and corrects the gear ratio to the low side (downshift direction). The running resistance increase amount ΔR is equivalent to the inclination resistance, and is calculated by subtracting air resistance, rolling resistance, and acceleration resistance from driving force of the driving wheels.
When the brake pedal is applied while driving downhill (during downhill control), the control device for a continuously variable transmission of JP-A No. 2003-83434 halts the downhill control, and the control state is held. Suppose, for example, that when downhill control is executed while driving downhill and the gear ratio is corrected to the low side (downshift direction), the brake pedal is depressed so that the downhill control is held, and then the grade of the road becomes less steep. In this case, the level of the held downhill control is excessive for the less steep downhill road. In such state, if the brake pedal continues to be depressed so that the vehicle speed drops, the gear ratio is shifted down to the low side with the engine speed being maintained as the vehicle speed decreases, whereafter the engine speed falls according to a low gear line in a gear characteristic property line diagram. Accordingly, the engine speed is maintained high, which may give the driver a feeling of wrongness.